1. Technical Field
This invention relates to the use of an anion selective compound in a membrane to measure the amount of a specific anion of choice in aqueous solution.
2. Background Art
Existing ion selective electrodes specific for anions typically have poor response characteristics including small linear ranges, high detection limits, and many interferences. Prior attempts to design such anion-specific electrodes have met with considerable difficulty. There is a need for a phosphate ion selective electrode having good selectivity and response characteristics employing existing electrode designs. With such a usable ion selective electrode, non-destructive techniques for the measurement of phosphate anions by an ion selective electrode can be applied to all areas of measurement science including environmental, medical and industrial applications.
Because phosphate anions are ubiquitous, an ion selective electrode selective for phosphate anions is particularly desirable. There have been numerous attempts to fabricate membranes suitable for such use. These attempts have included the formation of membranes from insoluble phosphate salts, and the incorporation of enzymes in the membrane to promote enzymatic reactions in the membrane. Unfortunately these systems have failed to yield anything but poorly responding electrodes with high detection limits or unsuitably high interferences from other common anions, thus rendering them of no practical use.
U.S. Pat. No. 3,857,777 to Guilbault et al, describes a phosphate selective electrode based on membranes containing a mixed complex consisting of silver, thiourea, and HPO.sub.4.sup.2-. Unfortunately, selective electrodes based on this system suffer from severe interferences such as Cl.sup.-, NO.sub.3.sup.-, and other abundant anions. Furthermore this electrode deteriorates rapidly and does not produce consistent results after three days.
U.S. Pat. Nos. 4,735,692 and 4,900,404, to Arnold et al, describe a group of organo-tin compounds selective to HPO.sub.4.sup.2- ions used to produce a membrane electrode selective to phosphate. These organo-tin compounds specifically bis(p-chlorobenzyl)tin dichloride are thought to complex phosphate anions preferentially. It was later determined that these electrodes suffer from severe interferences from other commonly occurring anions and that electrodes based on the bis(p-chlorobenzyl)tin dichloride are actually more sensitive to SCN.sup.- than phosphate. Additionally, electrodes based on these compounds have a linear range only to about 10.sup.-3 mol/l HPO.sub.4.sup.2- thereby rendering them unsuitable for many applications.
Thus prior attempts at producing phosphate anion selective electrodes have been ineffective due to interferences from other anions and have exhibited poor sensitivity. Clearly there is a need for an effective and reliable phosphate ion selective membrane for use in ion selective electrodes.